The present invention relates to a high-frequency power amplifier, and more specifically to a high-frequency power amplifier which comprises a transistor for high-frequency power, an input bias circuit connected with the input terminal of the transistor, an output bias circuit connected with the output terminal of the transistor, an input impedance matching circuit connected with the input terminal of the transistor, an output impedance matching circuit connected with the output terminal of the transistor, and a bias voltage generation circuit for generating direct-current bias voltage required in the input bias circuit.
The high-frequency power amplifier is used in the transmission unit of mobile communication equipment, and has a function to amplify a modulated signal so as to have a predetermined output power.
At present, the most common frequencies used for portable phones are 1 GHz to 2 GHz, and the output power of the high-frequency power amplifiers having these frequencies is about 20 dBm to 36 dBm. In order to produce high-frequency power in this range, a multistage high-frequency power amplifier, which has two or three stages and a gain of about 30 dB, is usually used. The multistage high-frequency power amplifier amplifies a modulated wave output of about 0 dBm so as to have a transmission output of about 30 dBm. The transmission output thus amplified is transmitted as an electric wave from the antenna.
The transistor for high-frequency power used in a high-frequency power amplifier can be a MOSFET, a bipolar transistor formed on a silicon substrate, a MESFET, or a heterojunction transistor including a FET formed on a semiconductor substrate containing a compound such as gallium-arsenic (GaAs). A general method of using such a transistor for high-frequency power under the conditions of few distortions and a favorable gain is to have a direct-current bias power supply connected with the input and output terminals of the transistor, and to have high-frequency power enter the transistor to start the amplification operation while the direct-current bias power supply supplies the transistor with a predetermined bias current,
A first conventional high-frequency power amplifier having the above-mentioned structure will be described as follows with reference to FIG. 15.
FIG. 15 shows the circuit structure of the first conventional high-frequency power amplifier. In FIG. 15, Tr.sub.a and Tr.sub.b represent a former-stage transistor for high-frequency power and a latter-stage transistor for high-frequency power, respectively, and their source terminals are grounded.
The input terminal (gate terminal) of the former-stage transistor Tr.sub.a is connected with an input impedance matching circuit 1 which consists of an inductor L and capacitors C. The input impedance matching circuit 1 matches the impedance of the high-frequency power entered from an external input terminal RF.sub.in and the input impedance of the former-stage transistor Tr.sub.a so as to reduce the reflection and the loss of the entered high-frequency power.
The input terminal of the former-stage transistor Tr.sub.a is connected with an end of a first microstrip line S.sub.1 having a 1/4 wavelength as a first input bias circuit, while a negative power supply -VGG.sub.1 is connected with the other end of the first microstrip line S.sub.1. The input terminal of the former-stage transistor Tr.sub.a is supplied with negative direct-current bias voltage through the negative power supply -VGG.sub.1 and the first microstrip S.sub.1. The negative power supply -VGG.sub.1 is connected with a bypass capacitor C.sub.b1 which suppresses the oscillation of high-frequency power, thereby preventing the high-frequency power from flowing towards the negative power supply -VGG.sub.1. As the first input bias circuit, the first microstrip line S.sub.1 having a 1/4 wavelength can be replaced by a resistance having several K.OMEGA..
On the other hand, the output terminal (drain terminal) of the former-stage transistor Tr.sub.a is connected with a positive power supply +VDD1, via a second microstrip line S.sub.2 having a 1/4 wavelength as a first output bias circuit, and is supplied with a positive direct-current bias voltage.
In the same manner as the former-stage transistor Tr.sub.a for high-frequency power, the input terminal of the latter-stage transistor Tr.sub.b for high-frequency power is connected with a negative power supply -VGG.sub.2, via a third microstrip line S.sub.3 having a 1/4 wavelength as a second input bias circuit, whereas the output terminal is connected with a positive power supply +VDD.sub.2 via a fourth microstrip line S.sub.4 having a 1/4 wavelength as a second output bias circuit.
An interstage impedance matching circuit 2 is connected between the output terminal of the former-stage transistor Tr.sub.a and the input terminal of the latter-stage transistor Tr.sub.b, so as to match the output impedance of the former-stage transistor Tr.sub.a and the input impedance of the latter-stage transistor Tr.sub.b. The interstage impedance matching circuit 2 consists of an inductor L and capacitors C, like the input impedance matching circuit 1. The interstage impedance matching circuit 2, which performs an impedance matching between the former-stage transistor Tr.sub.a and the latter-stage transistor Tr.sub.b in a multistage high-frequency power amplifier which has plural transistors for high-frequency power, has a function as an output impedance matching circuit for the former-stage transistor Tr.sub.a, and a function as an input impedance matching circuit for the latter-stage transistor Tr.sub.b. The output terminal of the latter-stage transistor Tr.sub.b is connected with an output impedance matching circuit 3, which consists of an inductor L and capacitors C. The output impedance matching circuit 3 matches the output impedance of the latter-stage transistor Tr.sub.b and the impedance of the high-frequency power outputted from an external output terminal RF.sub.out.
The first conventional high-frequency power amplifier amplifies the high-frequency power (a high-frequency signal) entered from the external input terminal RF.sub.in in the former-stage transistor Tr.sub.a and the latter-stage transistor Tr.sub.b, and then outputs the amplified high-frequency power from the external output terminal RF.sub.out.
The former-stage and latter-stage transistors Tr.sub.a and Tr.sub.b in the first conventional high-frequency power amplifier have the current-voltage characteristics shown in FIG. 16 (a). To be more specific, when the gate voltage V.sub.g is negative, for example -2 V, the drain current I.sub.ds hardly flows, whereas as the gate voltage V.sub.g gets closer to 0 V, the drain current I.sub.ds flows more. Consequently, in order to reduce the drain current I.sub.ds, it is necessary to provide a direct-current bias power supply which makes the gate voltage Vg about -1 V to -1.5 V.
Therefore, in the first conventional high-frequency power amplifier, the direct-current bias voltage of the former-stage and latter stage transistors Tr.sub.a and Tr.sub.b is set as follows. Each input terminal is connected with a negative power supply -VGG.sub.1 (-VGG.sub.2) having a value of minus several volts, whereas each output terminal is connected with a positive power supply +VDD.sub.1 (+VDD.sub.2) having about 6 V. Thus a drain current having a predetermined bias current, such as 100 mA, flows by means of these negative and positive power supplies.
However, the first conventional high-frequency power amplifier has a drawback concerning its troublesome bias current setting as follows. The amplifier needs two direct-current bias power supplies: a negative power supply -VGG.sub.1 (and/or -VGG.sub.2), which is a negative direct-current bias power supply and a positive power supply +VDD.sub.1 (+VDD.sub.2), which is a positive direct-current bias power supply. As a result, their structure and the peripheral circuits become complicated. In addition, the accuracy of the bias voltages of these direct-current bias power supplies must be set at 10% or below of the pre-determined bias voltage to prevent the impedance of the transistor Tr.sub.a (and/or Tr.sub.b) from changing, so as to secure impedance matches, satisfactory distortion characteristics, and gains.
The first conventional high-frequency power amplifier has another drawback concerning an increase in the operating current as follows. The direct-current bias current is made to flow constantly in the amplifier even when the output of the high-frequency power is small, thus increasing the power consumption. As shown in FIG. 16 (b), the operating current does not decrease very much when the output of the high-frequency power decreases.
In the case of a portable phone, for example, when it is far from the base station, the electric wave is sent out at the maximum output. Whereas when it is near the station, the electric wave is sent out at a low output. As a result, the waste of the batteries is avoided, making the batteries last longer. However, even when the electric wave is sent out with low high-frequency power near the base station, the same amount of idle current as is needed for the maximum output flows, which consumes the batteries.
In view of these drawbacks of the first conventional high-frequency power amplifier, a high-frequency power amplifier which changes the direct-current bias current in accordance with entered high-frequency power has been proposed in a thesis on page 1,533 in Electronics Letters Vol. 32, No. 17.
The high-frequency power amplifier disclosed in the thesis will be described as a second conventional high-frequency power amplifier as follows with reference to FIG. 17. In FIG. 17, the same components as those of the first conventional high-frequency power amplifier are assigned the same reference numbers, and their description will be omitted. The input impedance matching circuit 1, the interstage impedance matching circuit 2, and the output impedance matching circuit 3 are shown in the form of blocks for the sake of convenience. The voltage-current characteristics of the former-stage transistor Tr.sub.a and the latter-stage transistor Tr.sub.b are shown in FIG. 16 (a), in the same manner as those of the first conventional high-frequency power amplifier.
The second conventional high-frequency power amplifier, as shown by a chain line in FIG. 17, comprises a negative bias voltage generation circuit connected with the output terminal of the output impedance matching circuit 3. The negative bias voltage generation circuit comprises a detection circuit for high-frequency power including a resistance R.sub.1 and a diode D.sub.1, and a voltage division circuit including a negative direct-current bias power supply Vc (-5 V) and division resistances R.sub.2, R.sub.3, and R.sub.4. The negative bias voltage generation circuit detects part of the high-frequency power which is outputted from the output impedance matching circuit 3 at a power detection point E, changes the direct-current bias voltage which is outputted from the negative direct-current bias power supply Vc, based on the detected high-frequency power, and outputs the direct-current bias voltage to the input terminal of the latter-stage transistor Tr.sub.b from a negative bias voltage output point D via the third microstrip line S.sub.3.
According to the above-mentioned structure, when the high-frequency power detected at the power detection point E is large, the bias voltage which is outputted from the negative bias voltage output point D increases as high as -1 V, whereas when the high-frequency power detected at the power detection point E is small, the bias voltage which is outputted from the negative bias voltage output point D decreases as low as -5 V. This is how the idle current is reduced when the entered high-frequency power is small. As a result, as shown in FIG. 18, the operating current when the entered high-frequency power is small becomes lower than that in the first conventional high-frequency power amplifier shown in FIG. 16 (b).
However, the second conventional high-frequency power amplifier also has a drawback. The peripheral circuit has a negative direct-current bias power supply V.sub.c, and for the negative direct-current bias power supply V.sub.c to generate negative voltage of around -5 V, it is necessary to provide a negative voltage generation circuit which generates negative voltage based on the positive voltage outputted from the positive power supply +VDD.sub.2. Since the negative voltage generation circuit is constantly supplied with a current of 10 mA, the second conventional high-frequency power amplifier fails to reduce the current consumption when the entered high-frequency power is small.